1. Field of the Invention
The present invention relates to insulation systems used in thermal analysis instruments such as dynamical mechanical analyzers (DMAs) and other scientific or technical apparatus.
2. Background of the Invention
The present invention will be described primarily as it is used with dynamic mechanical analyzers (DMAs), as described in application Ser. No. 08/609,547, the parent of the present application, which is incorporated herein by reference. However, it may be incorporated in other apparatus for which the thermal sheet insulation system offers an advantage over conventional insulation systems, e.g., because the instruments will be used at sub-ambient temperatures, or because conventional insulation cannot tolerate the experimental conditions.
The sample and the fixtures in DMAs and TMAs are enclosed within a temperature-controlled sample chamber which can heat the sample and the fixtures to temperatures above normal ambient temperatures or cool the sample and the fixtures to temperatures below normal ambient temperatures. The temperature is generally varied dynamically, e.g., at a constant heating or cooling rate.
The sample chamber heats or cools the sample and fixtures, and provides a protective atmosphere to prevent sample degradation. Resistive heating elements can be located within the sample enclosure heating the sample and its fixtures directly, or they can be located external to the sample enclosure, heating air which is passed through the sample enclosure by a fan.
The sample and fixtures are cooled by introducing a cryogenic liquid or gas, generally nitrogen, into the sample chamber. When the cooling medium is a gas, the liquid cryogen is evaporated external to the sample chamber and the cold gas is transmitted to the sample chamber. When the cooling medium is a liquid, the liquid cryogen is transmitted to the sample chamber where it evaporates and cools the sample and its fixtures. Because the evaporation of a cryogenic liquid absorbs a large quantity of energy, a much greater cooling effect is available when using evaporation of the cryogen within the sample chamber, leading to a much lower consumption of the cryogen. Unfortunately, the difference in temperature between the liquid and the gas is large, so that the evaporation process can cause large temperature variations within the sample chamber, which in turn can cause large and erratic variations of the sample temperature.
Because DMAs are often operated at temperatures well below room temperature, condensation of atmospheric moisture within the sample chamber can occur. In most cases, the sample region is purged with a dry gas to prevent this moisture from contaminating the sample. Conventional DMAs use prior art fibrous thermal insulation to maintain the low or high temperature of the sample region. Although fibrous insulation is a very effective thermal insulator, it also absorbs moisture from atmospheric condensation very readily. When the sample chamber is cooled, this moisture freezes, forming ice which reduces the effectiveness of the insulation. Later on, when the DMA is heated up, the ice melts and may drip into the sample region and may contaminate the sample.